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Biology Unit 3.4: Cell Processes; Scientists Cheat Sheet by

Jan van Helmont (1600s)

He weighed a pot of soil before and after he planted a willow tree in it
He only gave it water for five years
The pot weighed the same afterwards but the tree was sevent­y-five kilograms so he concluded the plants mass came from the water and not the soil
He was right for the most part

Joseph Priestly (1771)

He put a lit candle in a jar with a piece of a mint plant
The candle soon went out (oxygen had not been discovered yet*) but he knew it "­dec­omposed the air" somehow
There was then no oxygen in the jar
He left the plant in the jar for a while and came back later
He took a magnifying glass and was able to light the candle through the jar, meaning somehow that there was now oxygen in the jar
He concluded then that plants changed the compos­ition of the air
*Oxygen was discovered in 1772

John Woodward (late 1600s)

He measured the amount of water he gave to his plant
He gave his plant 76,000 grams of water
However, the plant only grew one gram
So he concluded most of the water that was given to the plant was exhaled through its leaves

Jan Ingenhousz (1779)

He started out similarly to Priestly's experi­ment; with a lit candle and a plant in a jar
He then put a black cloth over the jar so light could not get into the jar
When he tried to light it later, the candle would not light
He proved that plants need light to change the compos­ition of the air
He also did another experiment where he had an aquatic plant
When there was light, the plant released little bubbles into the water
When there was no light, the plant did not produce bubbles
This further proved that plants need light to change the air


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